What does the line "And miles to go before I sleep" mean?

At a gathering in 1959 at the Waldorf Astoria hotel, honoring Robert Frost's eighty-fifth birthday, the eminent critic, Lionel Trilling, introduced Frost by stating,"...He is not the Frost who reassures us by his affirmation of old virtues, old simplicities and ways of feeling; he is anything but. Frost's best poems represent the terrible actualities of life, he is a terrifying poet." Frost, for all his folk wisdom and rural imagery, weaves throughout his work a tone of unease, which leaves the reader with a sense that this world is not as it would appear to be.
Throughout the poem, "Stopping by the Woods on a Snowy Evening," Frost paints a picture of what seems a calm pause in a long journey. But on closer reading, there are tense questions asked, as the rider brings his horse to a full stop in the middle of the woods. The obvious fist reading seems a simple tale of a rider stopping to take in the beauty of the quiet, snowy, dark woods. But in a deeper reading, it is the last two lines that change the tone of the poem from bucolic to unnerving. "But I have promises to keep/ and miles to go before I sleep/miles to go before I sleep." This still moment in time cannot be held onto. The rider has stopped in a dark woods, alone with only the horse that carries him, and in the repetition of the last lines, the rider has reminded himself that this dark journey is long, as life is long and that he cannot stop just here, but must continue to the very end. Eternal sleep, but miles to go.

The Robert Frost poem "Stopping by Woods on a Snowy Evening" paints a beautiful word picture of a wooded country road on a winter's evening while it is snowing. The speaker mentions in the first stanza that the owner of the woods lives in the village and would not see him stopping here, ending with the brilliantly haunting line "To watch his woods fill up with snow."
The short poem continues with thoughts that the horse pulling his sleigh or wagon (the poem does not specify which) must think it queer that they have stopped in the woods, with no farmhouse near.
The last stanza of the poem is where the "And miles to go before I sleep" line is written, and it is written twice for emphasis. The speaker has just stated that "The woods are lovely, dark and deep. / But I have promises to keep."
Ending the poem with the words "And miles to go before I sleep. / And miles to go before I sleep" emphasizes the fact that the speaker realizes he is not close to home, where he can sleep. I believe that the repetition provides the idea that the speaker is likely repeatedly thinking about how far he must go that night.